i think you have missed a few many updates of Manager. the bank and cash accounts were recently split to their respective tabs. you can read about it in the below topic.
if you had initially created the account and missed to tick a check box which identifies the account as managed by a financial institution, then the chances are that the above mentioned update would have moved your account to Cash Accounts tab.
if the account is actually a cash account, you do not have to do anything more and can continue using as it is. but if it is a bank account maintained by a financial institution, then you can just convert the account. read this guide https://www.manager.io/guides/11727